This invention relates to motorcycles, and more particularly, to motorcycle suspensions.
Prior art motorcycles generally include a frame for supporting an engine and transmission and a rear wheel suspension which absorbs road shocks to provide the operator with a more comfortable ride. Such prior art motorcycle rear wheel suspensions generally include a rear fork or swing arm pivotally mounted at one end on the frame and which rotatably supports the rear wheel at the other. In addition, a spring/damper may be provided between the motorcycle frame and the swing arm. Such motorcycles are commonly driven by a belt or chain extending from a drive sprocket coupled to the transmission to a driven sprocket mounted on the rear wheel. Motorcycle drive belts normally comprise a flexible member having molded teeth while drive chains comprise a continuous belt of pivotally connected metallic links. For purposes of brevity, the term belt or belt means as used hereinafter is defined as either a belt or a link chain as used in motorcycle drive assemblies.
Drive belt life is directly related to the ability to maintain accurate matching of belt and sprocket teeth. This can be ensured only when the center distance between drive and driven sprockets is prescribed and maintained. Prior art designs have effected this by concentrically pivoting the swingarm about the transmission output shaft and drive sprocket rotational axis. Design complexity, design packaging, and serviceability access issued have prevented wide spread use of this design in production motorcycles. The most often employed method of swingarm pivoting and mounting is in a manner non-concentric with the drive sprocket.
As the non-concentrically mounted swingarms of prior art motorcycles pivot under the influence of road shocks, the distance between the centers of drive and driven sprockets varies thereby disallowing consistent matching or meshing of belt and sprocket teeth. This adversely affects belt life. Moreover, changes in tension of the belt or chain adversely affects the smoothness of the ride.
Uniform feel and handling characteristics during vehicle acceleration of prior art motorcycles, with non-concentric swingarm mounting, is adversely affected by excessive variation in the degree of suspension "anti-squat". Anti-squat is defined as the force imparted to the vehicle, through drive belt tension and rear suspension geometry, which resists rear suspension vertical travel or "jounce" due to weight transfer under vehicle acceleration. Vehicle acceleration is directly affected by the traction or tractive effort available. Tractive effort is directly proportional to the vertical force between the road surface and the drive tire patch. Reduction in the degree of anti-squat during suspension displacement adversely affects the increase in the tire patch force and consequential traction as weight is transferred to the rear wheel under acceleration. Reduction in available traction as the vehicle suspension undergoes acceleration induced jounce, reduces uniformity of vehicle feel and handling characteristics.